Electron discharge apparatus of the beam type



2 Sheets-Sheet 1 R. L. JEPSEN ELECTRON DISCHARGE APPARATUS OF THE BEAMTYPE Aug. 15, 1961 Original Filed Oct. 5. 1955 DISTANCE r I E- a A E09521- L F /D X A 7702 NE Y DISTANCE :Elrj A R. L. JEPSEN Aug. 15, 1961ELECTRON DISCHARGE APPARATUS OF THE BEAM TYPE 2 Sheets-Sheet 2 OriginalFiled Oct. 5, 1953 r1 E E rIlI5 56 L T 3 T 3 2? 6/ DISTANCE" INVENTOREaae'zr-L. c/PSEN FIE 1' A 2,996,639 ELECTRON DISCHARGE APPARATUS OF THEBEAM TYPE Robert L. .Iepsen, Los Altos, Calif., assignor to VarianAssociates, San Carlos, Calif., a corporation of California Originalapplication Oct. 5, 1953, Ser. No. 384,018, now Patent No. 2,888,599,dated May 26, 1959. Divided and this application Apr. 13, 1959, Ser. No.805,999

1 Claim. (Cl. 315--5.19)

This invention relates, generally, to ultra high frequency electron tubedevices or apparatus and the invention has reference, more particularly,to novel apparatus of this type which removes ions from the beam pathsthrough the devices, such as klystrons, traveling wave and otherelectron beam tubes, to thereby substantially eliminate perturbations ofthe beams thereof, and also to novel velocity modulation tubes of thereflex klystron type which are so contsmcted as to prevent reflectedelectrons from returning to the cathode region. This application is adivision of my United States patent application Serial No. 384,018,filed on October 5, 1953, entitled Electron Discharge Apparatus, whichhas now issued as Patent No. 2,888,599 on May 26, 1959.

Several difliculties encountered in electron discharge devices such as,for example, klystrons and traveling wave tubes heretofore employedarise from an intraction between the electron beam and ions trappedalong the path of the beam. These difficulties may include:

(1) Modulation of output power, frequency, and beam current at thefrequency or frequencies of continuous ion oscillation. Thesefrequencies are, typically, in the range .1 megacycleper second tomegacycles per second.

(2) Modulation of output power, frequency and beam current at both (a)Ion oscillation frequencies (in the megacycles per second range) and (b)Low frequencies (typically in the audio frequency range). The lowfrequency fluctuations are associated with the high frequency ionoscillations.

(3) Changes in output power, frequency, and beam current as thefrequency of an applied modulating voltage is varied. These changesoccur when the modulating frequency is near one of the frequencies ofincipient ion oscillation and they may be abrupt or fluctuating. In manyheretofore existing reflex ldystrons a source of trouble encountered isthe occurrence of what has been termed multiple transits of the beam.Multiple transits refer to the action of the electron beam after passingthrough the resonator gap subsequent to being reflected back to thenegatively charged reflector. After this reflection or return tripthrough the resonator gap, the bunched electrons have performed theiruseful function and their elimination from further operation on the tubeis desirable. However, many of the returning electrons enter the cathoderegion of the reflex klystron and are turned about by the negativecharge to begin a second round trip through the resonator gap. Thesemultiple transits of the electrons produce undesirable effects in thetube such as, for example, unstable power output.

One object of the present invention is to provide a novel method andmeans in electron beam discharge devices for preventing the trapping ofions in the beam path.

Another object of this invention is to provide a novel method and meansfor draining ions out from spaces defined by grided apertures.

These and other objects and advantages of the present invention will bebetter understood from perusal of the following description andexplanation of the drawings wherein arcane Patented Aug. 15, 1961 FIG. 1is a schematic elevation view in section of a refiex klystron whichembodies the present invention whereby multiple transits of theelectnons in the tube are prevented and whereby the ions are continuallydrained from the re-entrant tube or drift space between the cathode andthe first resonator grid to prevent the formation of an ion traptherein.

FIG. 2 is a section view of the reflex klystron of FIG. 1 taken alongsection line 2-2.

FIG. 3 is a section view of a portion of a reflex klystron of the typeheretofore employed wherein ions are trapped in the drift space orre-entrant tube portion of the tube between the cathode and the firstresonator grid.

FIG. 3(a) is a plot of the potential along the beam axis versus thedistance along the re-entrant tube space for the reflex klystron shownin FIG. 3 showing the formation of an ion trap.

FIG. 4 is a section view of a portion of a reflex klystron whichembodies the present invention wherein the walls of the re-entrant tubespace between the cathode and the first resonator grid are so formed asto prevent the formation of ion traps therein as occurs in the klystronof FIG. 3. This novel feature is utilized in the klystron of FIG. 1.

FIG. 4(a) is a plot of the potential along the beam axis versus thedistance along the beam path in the reentrant tube space of FIG. 4showing how the formation of ion traps is prevented.

FIG. 5 is a section view of a portion of a reflex klystron substantiallyidentical to that shown in FIG. 1 which embodies a novel additionalfeature to the klystron of FIG. 1 whereby the ions in the resonator gapspace are also continually drained therefrom to prevent the formation ofan ion trap between the FIG. 6 discloses in section view a two-cavityklystron employing an annular shaped beam of electrons wherein a novelmethod and means is provided for continually draining ions from there-entrant tube or drift space between the accelerating grid and thefirst resonator grid to prevent the formation of an ion trap therein.

FIG. 7 shows in longitudinal section view a portion of a three-cavityVelocity modulation tube which embodies the novel invention forpreventing the formation of ion traps at the resonator gaps as occurredin heretofore employed velocity modulation tubes of this type.

FIG. 7(a) is a plot of the potential along the beam axis versus thedistance along the beam path for the novel beam device shown in FIG. 7and also includes a plot of the beam potential along the axis of a beamversus the distance along the beam path for the heretofore employedklystrons of this general type to more clearly illustrate theimprovement brought about by the invention disclosed in FIG. 7.

Referring now to FIGS. 1 and 2 there is shown a reflex klystronembodying the present invention, there being shown only such parts of areflex klystron as are necessary to illustrate the present invention. Itshould be understood that the beam path of the particular klystron wouldalso be enclosed in a vacuum envelope or body and the klystron wouldinclude other elements such as, for example, an output window. Thereflex klystron comprises an annular cathode I having a concave orrecessed emitter surface, the cathode surface sloping radially inwardlyand downwardly as viewed in FIG. 1 to form substantially an acute anglewith respect to the axis of the klystron tube. .An annular heater 2 isshown positioned on the under side of the cathode to provide thenecessary heat for electron emission. An annular, substantiallycupshaped focusing ring 3 encircles the annular cathode on both itsouter and inner peripheries to provide for focusing of the electron beamemitted from the cathode. Located within the center or frame opening inthe annular focusing ring and cathode and in axial alignment therewithand also extending slightly above the cathode emitter surface is ametallic collector electrode 4. Positioned in axial'alignment with thecathode focusing ring 3 and collector 4 is a cavity resonator 5including a pair of mutually spaced resonator grids 6 and 7. The driftspace or re-entrant tube formed by the outer wall 8 of the re-entrantportion of the cavity resonator is of substantially truncated coneconfiguration, the sides of which have a slightly convex shape, thisparticular configuration being employed for a purpose to be subsequentlydescribed. Axially aligned with the grids 6 and 7 and positioned abovegrid 7 is a metallic concave reflector 9.

In operation, an annular shaped beam of electrons is emitted from thecathode 1 and is focused into a beam by the focusing ring 3, the streamof electrons being accelerated toward the resonator grids 6 and 7 by thepositive potential of the cavity with respect to the cathode as providedby the source of potential represented by battery 11. Because of theangle of the emitter surface of the cathode 1 with respect to the axisthrough the tube, the annular electron beam is directed in such a manneras to tend to form a hollow substantially conical beam, the apex ofwhich coincides with the axis of the klystron and is located within there-entrant tube portion 8, the sides of the beam being somewhat concaveand conforming to the convex surface of the tube re-entrant portion 8.However, the electrons in the annular beam, as they approach the axis,repel each other due to space charge and this interaction between theelectrons tends to bend the annular beam within the re-entrant tubeportion so as to form the beam into a straight substantially hollowcylindrical beam. The electrons in this hollow cylindrical beamconfiguration pass through the cavity resonator gap between the grids 6and 7 with the electrons traveling in paths perpendicular to the gridsor, as stated in another way, parallel to the electric field vectorsacross the resonator gap. The electrons are velocity modulated by theradio frequency voltages across the gap in a well known manner and arerepelled by the reflector 9 which may carry a negative potential withrespect to the cathode. The electrons are turned about and again passthrough the resonator gap in bunches parallel to the electric fieldvector across the gap to give up energy to the field in the cavityresonator. The stream of electrons continues axially through there-entrant tube 8 and are collected on the collector 4 which isconnected to a source of potential positive with respect to the cathode,the potential of which may be made variable as represented by thevariable resistor 12. As noted, the returning electrons are in the formof a cylindrical beam and, since the diameter of this beam is determinedin part by the diameter of the resonator grids, the collector electrodeupper surface is shown having a diameter equal to or slightly largerthan the diameter of the grids. Since the returned electrons are allcollected on the positive collector electrode 4, few, if any, willre-enter the region of the negatively charged cathode 1 where they wouldbe turned about and started on another trip through the tube and thusthe problem of multiple transit is eliminated. A recess or pocket 13 isshown in the upper surface of the collector 4 and serves the purpose ofretaining any secondary emission electrons which may be emitted from thecollector by the striking electrons. A decided advantage in the use ofthe collector electrode for catching the electrons after they haveperformed their useful function in the tube is that the electrons arecollected on a separate electrode, the collector, rather than on thewalls of the tube and the cavity resonator, and thus the heat generatedby the striking electrons does not cause expansion of the cavityresonator or other critical tube parts with a resultant change in theoperating frequency of the tube.

The collector is connected to a source of potential slightly negativewith respect to the potential of the cavity resonator by the variableresistor 12. The collector, be

cause of its slight negative potential, attracts the positive ions whichare produced in the re-entrant tube or drift space 8 of this klystrondue to the collision of the electrons with gas molecules in the space.The collector thus continually drains the ions from the space andprevents the formation in the beam path of a deleterious ion trap. Theparticular configuration of the re-entrant tube walls 8 also serves toprevent the formation of an ion trap as will be more readily understoodfrom the following description of this novel feature disclosed in FIG.4.

In heretofore existing reflex klystrons wherein attempts were made toprevent the beam from returning into the cathode region after passingthrough the resonator gap, one example of which employs a spike in thecenter of the reflector to produce an umbrella-shaped reflected beam,the electron beam does not travel parallel to the electric field vectorsacross the resonator gap during both passages thereacross and,therefore, the optimum interchange of energy between the beam andresonator field does not occur. In this present invention, the electronbeam travels parallel to the electric field vectors across the resonatorgap on both passages therethrough to give the maximum energy exchangeeven though the electron beam does not re-enter the negative cathoderegion on its return trip through the klystron.

Referring to FIG. 3 there is shown a portion of a reflex klystron tubeof the heretofore employed type including the cathode 16, the focusingring 17, the walls 18 forming the cylindrical re-entrant or drift spaceand the first resonator grid 19. The outer periphery surface of the beamemitted from the cathode and focused by the focusing ring is shown indotted lines. It is noted that at the entrance to the re-entrant tubespace the beam oecupies substantially the entire opening, the beamoccupying a progressively smaller portion of the cylindrical drift tubespace as it proceeds toward the resonator grid 19. The beam may spreadagain as it approaches the grid 19 but, in any case, the beam occupies asmaller proportion of the drift space at some point within the spacedefined by Walls 18 than at the left-hand end of the drift space. Thisparticular relationship between the walls of the drift tube space andthe beam shape produces an ion trap within the drift tube. The amount ofpotential depression within a drift tube due to the passage of anelectron beam through the drift tube depends on the beam voltage, on thebeam current, on the distribution of current across the beam, and on thegeometry of the drift tube. In particular, increasing the diameter of acylindrical drift tube, keeping all other quantities constant, resultsin an increased potential depression.

This ion trap is better illustrated in FIG. 3(a) which is a plot of thepotential along the beam axis between the entrance to the re-entranttube 18 and the resonator grid 19. The ion trap is formed where thepotential curve drops below the positive value which is present at theleft-hand end of the re-entrant tube or drift space due to the spacecharge effects of the electron beam. This ion trap is represented by thecross-hatched area 21. The positive ions in excess of the numbernecessary to fill the trap or, in other words, to balance the decreasein positive potential of the beam will drain out the left-hand end ofthe re-entrant tube. The present inventors have devised a novel drifttube structure which prevents the formation of ion traps, this novelstructure employed in FIG. 1 is being shown in FIG. 4.

Referring to this FIG. 4 and also to FIG. 4(a) it will be noticed thatthe drift tube space, rather than being of a cylindrical shape, is nowof an approximately truncated cone shape with convex walls, the wallstapering toward the right-hand or grid end. The cathode 22 and focusingring 23 may be of the same construction as shown in FIG. 3 and the beamproduced thereby of the same shape as that in FIG. 3. The drift tubewalls are so shaped with relation to the beam perimeter that as the beamprogresses toward the resonator grid 24 it occupies a progressivelyincreasing portion of the drift tube space. This particular relationshipbetween the drift tube walls and the electron beam prevents theformation in the beam path of any ion traps. This is better illustratedin FIG. 4(a) which shows a plot of the potential along the beam axisbetween the left-hand end of the re-entrant tube and the resonator grid.As can be seen there is no depression or decrease in the positivepotential along this drift space length so that substantially all of thepositive ions formed in the drift tube space drain out the left-hand endthereof.

It is this particular feature of the re-entrant tube portion of theklystron in FIG. 1 which aids in preventing the formation of ion trapsin the re-entrant tube space of this tube as mentioned above.

Referring to FIG. 5 there is shown therein in section view a portion ofa reflex klystron tube of the type shown in FIG. 1, there beingdisclosed only the reflector electrode 26, the two resonator grids 27and 28 and a portion of the walls 29 defining the re-entrant tube andcavity resonator. It will be noted that the second resonator grid 27 isthinner than the first resonator grid 28 and also that the openings orinterstices defined by the vanes of the grid 27 are substantially largerthan those of the first resonator grid. This particular type of secondresonator grid permits a portion of the negative field produced by thereflector electrode 26 to penetrate through this grid into the resonatorgap space between the two grids, thus providing for the draining ofpositive ions from the resonator gap to the reflector and therebypreventing the formation in the resonator gap of an ion trap.

In FIG. 6 is disclosed still another novel structure for use in electronbeam apparatus for draining ions fro-m drift spaces. There is shown justthat much of a twocavity resonator klystron which suffices to explainthe present invention, it being understood that the apparatus shown, orat least the beam path, is enclosed within a vacuum type envelope andmay include other structural features not shown. An annular cathode 31having a slightly concave emitter surface produces an annular beam ofelectrons when heated by a heater 32 extending under the cathode. Thisstream of electrons is focused by a circular focusing member 33 having asubstantially W-shaped cross section, the upwardly extending cylindricalcentral portion 34 of this focusing member extending within the centralopening of the annular cathode 31 and in axial alignment with theklystron. An annular accelerating grid 35 is positioned in the path ofthe electron beam for accelerating the electrons therein to a constantvelocity within the drift space formed by wall 36 of the tube. The beampasses through the gap formed by the resonator grids 37 and 38 of thefirst or buncher cavity resonator 39 where radio frequency energy actson the beam for velocity modulating the electrons therein, the electronsthen passing through the drift space 64 where they become densitymodulated and then passing across the second resonator gap formed by theoutput resonator grids 41 and 42 where the bunched electrons give upenergy to the output cavity resonator 43. The electrons in the beam thencollect on the collector electrode 44. Since in customary operation ofbeam discharge tubes of this type the focus electrode 33 is at anegative potential with respect to the walls of the drift tube space andthe cavity resonators and since in this particular embodiment thecentral upwardly extending portion 34 of the focusing electrode 33extends slightly above the cathode 31 and is aligned with the centralopening in the annular accelerating grid 35, ions will be drained outfrom the drift tube space formed by wall 36 by the negative potential onthe focus electrode through the central opening in the annularaccelerator grid to thus prevent formation of ion traps in the drifttube space.

FIG. 7 discloses another embodiment of the present invention wherein theion draining feature disclosed in FIG. 4 is applied to .a multicavitygridless klystron tube which may be, for example, of the high poweramplifier class such as disclosed in the U.S. patent application, SerialNo. 370,568, of Wayne G. Abraham and Sigurd F. Van'an entitled HighFrequency Tube, filed on July 27, 1953, now Pat. No. 2,879,440. Theelectron beam emitted from the cathode 51 is transmitted down the seriesof drift tubes 52, 53, 54 and 55, the ends of which define resonatorgaps in the three cavity resonators 56, 57 and 58. The inner diameter ofeach succeeding drift tube along the beam path is slightly smaller thanthe inner diameter of the preceding drift tube, the drift tube innerdiameters of each drift tube being constant. The electrons in the beamare expended in the collector end of the tube. In heretofore employedvelocity modulation tubes of this type the drift tubes had equal andconstant inner diameters throughout the length of the tube as shown inthe above cited patent application. In these heretofore employedmulticavity resonator tubes a potential depression is produced at eachof the resonator gaps, these depressions producing ion traps at eachgap. This is better illustrated in FIG. 7(a) which is a plot of thepotential along the beam axis versus the distance along the beam path.The potential curve labeled 61 is typical of the heretofore employedmulticavity lnlystrons while the curve 62 which is vertically transposedon the graph with respect to curve 61 is that of the novel tubestructure shown in FIG. 7. As seen in curve 61, the potentialdepressions at the reasonator gaps produce ion traps 63 which aredesignated by the cross-hatched area. As indicated by the curve 62representing the novel structure shown in FIG. 7, thme ion traps areeliminated due to the fact that as the curve is traced from left toright there occur no potential depressions where ion traps may beformed. In addition to preventing the formation of ion traps at theresonator gaps, this novel configuration also permits the rapid drainingof ions: formed throughout the entire length of the beam path due to thefact that the beam potential curve 62 decreases in its positive valuefrom right to left, whereas the beam potential curve 61 is a flat orunchanging curve. This illustrates another embodiment where an increasedportion of the drift space is occupied by the electron beam as it passesthrough the tube.

Although the klystron disclosed in FIG. 7 has each succeeding drift tubeof a smaller inner diameter than the preceding drift tube, the greatestbenefit in tube operation is derived from the elimination of the iontrap at the first resonator gap and a lesser benefit by eliminating theion trap at the second resonator gap. This is due to the fact that anyambiguities in operation. that appear early in the stage of thisamplifier rtube are amplified to a great extent as the beam proceedsdown the tube. If such a high degree of perfection in operation is notnecessary, the drift tube 55 may have the same sized inner diameter asthe drift tube 54 or, to carry it a step further, the inner diameters ofdrift tubes 53, 54 and 55 may be of the same value or drift tubes 54 55may be made with larger inner diameters than drift tube 53 in certaincases, for example, to accommodate spreading of the beam. In theselatter examples, ion traps would occur at the resonat/or gaps inresonators 57 and 58 but not at the resonator gap in resonator 56. Theselatter examples were given to illustrate the flexibility of thisparticular embodiment of the invention.

As an example of the application of this invention, the inner diametersof the drift tubes of a power amplifier tube of the type disclosed inthe above cited patent are each 1.2". To achieve the improvementdescribed herein this power amplifier tube would be modified such as,for example, changing the inner diameter of the first drift tube to1.5", the inner diameter of the second drift tube to 1.3" and the innerdiameter of the third and fourth drift tubes to 1.1".

example, its. utilization in other types of klystrons, in

traveling wave tubes and in other electron beam devices,

it is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeand not in a limiting sense.

What is claimed is:

In an electron discharge device comprising an annular cathode foremitting a beam of electrons, a focusing electrode for focusing the beamof electrons comprising a central portion axially aligned with thecentral opening in the annular cathode, a beam-field interactionstructure in the path of said electron beam, an annular accelerator gridaxially aligned with the cathode and positioned between said cathode andsaid interaction structure for accelerating the beam of electrons, thecentral portion of the focusing electrode being in axial alignment withthe central opening in the annular grid, a source of electricalpotential and means coupling the source to the discharge device suchthat the focusing electrode is at a negative potential with respect tothe accelerator grid to thereby cause positive ions formed in the regionbetween the accelerating grid and the interaction structure to bedrained through the central opening in the grid to the central portionof the focusing electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,449,569 Varian Sept. 21, 1948 2,498,673 Goudet et a1 Feb. 28, 19502,758,245 Valian Aug. 7, 1956 2,825,842 Kenyon Mar. 4, 1958

